1. Field of the Invention
This invention relates to a method and apparatus for blackening processing for a region outside of an irradiation field, wherein a blackening process for a region outside of an irradiation field is carried out on a radiation image, which has been recorded by use of a collimation plate and has an irradiation field thereon. This invention also relates to a template matching method and device for carrying out matching of an image, in which an object pattern having a predetermined shape is embedded, and a template, which has a contour corresponding to the predetermined shape of the object pattern, with each other.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as such techniques, the applicant proposed various radiation image recording and reproducing systems which use stimulable phosphor sheets.
When a radiation image of an object, such as a living body, is recorded on a recording medium, it is desirable that adverse effects of radiation upon the living body due to irradiation to object portions not related to a diagnosis, or the like, can be kept as small as possible. Also, if object portions not related to a diagnosis, or the like, are exposed to radiation, the radiation will be scattered by such portions to the portion that is related to a diagnosis, or the like, and the image quality will be adversely affected by the scattered radiation. Therefore, when a radiation image is recorded on the recording medium, a collimation plate is often used in order to limit the irradiation field to an area smaller than the overall recording region of the recording medium, such that radiation may be irradiated only to that portion of the object, the image of which is to be used.
In cases where a radiation image is recorded on a recording medium by using a collimation plate, a region outside of the irradiation field is not exposed to the radiation. Therefore, in cases where the reproduced visible image is displayed on a CRT display device, the luminance of the region outside of the irradiation field becomes high on the visible image. Also, in cases where the visible image is reproduced on photographic film, the image density of the region outside of the irradiation field becomes low on the visible image. Therefore, when the reproduced visible image is seen, strong light comes from the region outside of the irradiation field on the visible image and enters the eyes of the person, who sees the visible image. Accordingly, even if the image within the irradiation field has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness, the reproduced visible image gives a glaring feeling to the person, who sees it, and becomes hard to see.
In order to eliminate the aforesaid problems, the applicant proposed, in Japanese Unexamined Patent Publication No. 3(1991)-98174, an image processing method and apparatus, wherein image signal values representing a low level of luminance or a high level of image density (ordinarily, the lowest level of luminance or a highest level of image density) are allocated to the image signal values corresponding to the region, which is outside of the irradiation field. The processing for rendering the image density of the region outside of the irradiation field high (or rendering the luminance of the region outside of the irradiation field low) and thereby reducing the glaring feeling is ordinarily referred to as the high image density forming processing for the region outside of the irradiation field (or the blackening process for the region outside of the irradiation field). The term "low level of luminance" as used herein is employed in the cases where a visible image is displayed as a luminance distribution on a display screen of a CRT display device, or the like, i.e. a luminance display surface. Also, the term "high level of image density" as used herein is employed in the cases where a visible image is reproduced as an image density distribution on photographic film, or the like.
In order for the blackening process for the region outside of the irradiation field to be carried out, it is necessary to discriminate an irradiation field and the region outside of the irradiation field from each other with respect to a given radiation image. As techniques for recognizing an irradiation field, there have been employed the techniques comprising the steps of (a) setting a plurality of radial lines, which extend from a predetermined point contained in an irradiation field on an image toward ends of the image, (b) detecting an edge candidate point, which is considered as being located on a boundary (a contour) of the irradiation field, in accordance with image signal values corresponding to picture elements lying on each of the radial lines, a plurality of edge candidate points being thereby detected with respect to the plurality of the radial lines, and (c) recognizing the irradiation field in accordance with the thus detected edge candidate points. Such techniques are proposed in, for example, Japanese Unexamined Patent Publication No. 63(1988)-100437 and U.S. Pat. No. 4,967,079.
The proposed techniques are the techniques for automatically recognizing the irradiation field. However, in cases where the automatic recognition of the irradiation field is carried out, it often occurs that an incorrect region is recognized by mistake as the irradiation field. Also, since collimation plates having various different shapes are utilized, the problems occur in that, in order for the automatic recognition of the irradiation field to be carried out with respect to the collimation plates having various different shapes, algorithms cannot be kept simple, and the time required for the processing cannot be kept short.